Many electronic devices in a computer system need to be driven to an initial condition prior to turning on or resetting the computer system. An example of such an electronic device is a memory device. There are several types of memory devices, which are integrated circuit chips that may be packaged in a number of different ways. One type of memory is random access memory (RAM), memory that is typically used as main memory in the computer system. Most types of RAM memory are volatile, meaning that it requires a steady flow of electricity to maintain its contents. Therefore, the data stored in RAM is lost when the power is turned off. Another type of memory is FLASH memory, which is a type of non-volatile memory that retains the stored information even when the device is not powered. Each cell of the FLASH memory includes a floating gate field-effect transistor capable of holding a charge. The cells can be electrically programmed by charging the floating gate, and erased by removing the charge. The data in each memory cell is determined by the presence or absence of the charge in the floating gate. A typical memory device uses a variety of logic circuits such as latches, gates and flip-flops to support its operation. All of these logic circuits need to be driven to an initial condition, such as to a particular voltage during power-up in order for reliable operation of the device to occur.
Power-up generally refers to the ramping up of power from a nominal voltage (e.g., zero volts or some standby voltage) to a supply voltage. For an electronic device to begin proper operation, it should be properly powered-up. Power-up may occur at initial start-up of the device, but it may also occur sometime after initial start-up, such as after a dormant period where the power supplied to the device's internal circuits has been diminished to a standby level.
The device may include a means for generating an internal supply voltage that is regulated and independent of fluctuations in an external supply voltage. Having a reliable internal power source is necessary for achieving a proper power-up sequence, particularly in small portable devices, such as hand-held systems, for example, a hand-held flash memory-based digital camera. Typically during power up, the internal supply voltage is generated up to a level that is below the external supply voltage, one that is appropriate for properly operating the host device in a stabilized state without encountering the effects of voltage swings inherently found in the external supply voltage.
For a variety of reasons, a power level for active operation may be required during the power-up sequence, particularly in multi-chip packaging. For example, device parameters may need to be loaded in order to achieve optimized chip operation, or a power-on read may be required for system boot sequencing. During power-up the device typically draws current if the device is in active operation. In the case of multi-chip packaging, since each chip consumes current to be in active operation, executing chip initialization at the same time for all the devices in the package may result in too much consumption of power from the power supply. If each device that is initializing reaches peak current consumption at about the same time, the system may encounter a brown-out condition due to a sudden drop in the voltage of the power supply and one or more devices may power-up incorrectly. Consequently, errors may occur during chip operations, such as during boot-up, and prevent the system from operating as intended.
There is, therefore, a need for a computer system that reduces the consumption of peak current during power-up, particularly in the case of multi-chip packaging.